The IP Multimedia Subsystem (IMS), as defined by 3GPP and 3GPP2, merges telephony and Internet technology by providing an all-IP based architecture for the telecommunications industry. The IMS is based on the Session Initiation Protocol (SIP) and makes heavy use of the protocols defined within the Internet Engineering Task Force (IETF). The system offers a network of servers and databases that assist a user agent with the task of establishing and managing sessions. IMS uses the term “sessions,” because the connections between users are no longer limited to voice services (i.e., a telephone call). Sessions may be voice, video, text, or other services connecting two or more user agents together.
Referring now to FIG. 1, the IMS architecture 100 includes User Equipment (UE) 125, Call Session Control Functions (CSCF) 115, Home Subscriber Server (HSS) 120 and Application Servers 105 and 110.
The UE includes a device that contains the SIP User Agent that will initiate or terminate sessions.
The CSCFs 115 are responsible for managing the sessions, including security and interconnect. There are three types of CSCFs. First, a Proxy CSCF (P-CSCF) resides at the edge of the network and serves as an entry point for the UE into the IMS core. Second, an Interrogating CSCF (I-CSCF) serves as an entry point into the network for peering networks. The I-CSCF also acts as the lookup function for finding the appropriate serving node for a subscriber. Third, a Serving CSCF (S-CSCF) is responsible for authenticating the UE and managing ongoing sessions for the UE, including invocation of applications. The S-CSCF communicates with the HSS in order to retrieve the UE authentication information. After the user has been authenticated, the S-CSCF again communicates with the HSS 120 to retrieve the user profile which specifies the services to which a user has subscribed and which applications servers are to be invoked for those services.
The HSS 120 stores the relevant user data, including authentication information and service data. As part of the user profile, initial Filter Criteria (IFC) are defined to indicate which application servers are to be invoked, based on information in the signaling plane.
Applications Servers 105, 110 are invoked based on the iFC that are stored in the user profile. The S-CSCF will pass signaling onto an Application Server if the criteria defined in the IFC are met. Once invoked, the application server can take part in the session and provide additional capabilities.
Referring now to FIG. 2A, the architecture 200 of the converged CDMA IMS Femtocell system, as shown in FIG. 1 of section 5.1.1 of 3GPP2-X.S0059 Femtocell Overview, is illustrated.
The converged CDMA IMS Femtocell system includes a CDMA Femtocell Access Point (FAP) 238, which is a CDMA 2000 1x access point that provides coverage in a small area, usually a private residence or a small office, and connects a mobile station (MS) 240 to an operator's network 228 via a broadband connection, such as, for example, DSL or cable. Typically, a residential FAP is a miniature base station that implements the CDMA base transceiver station function, the base station controller function, and the packet control function (PCF). A plain old telephone service (POTS) telephone can be connected to a FAP via an ATA port. A typical residential FAP has a range of approximately 15 feet.
Referring also to FIG. 2B, the converged CDMA IMS Femtocell system also includes a cluster 238 of up to n FAPs in large enterprises, where n=a maximum number of base stations supported in a soft handoff scenario in IS-95 or IS-2000. One FAP in the cluster is the Controlling FAP (CFAP) 272, and other individual FAPs may be designated as FAP1 270 and FAPn 274. In a macrocell-to-femtocells active mode handoff scenario, an individual FAP other than the CFAP 272 sends voice packets to the CFAP 272, which selects the strongest voice frame to send towards the IMS 224 via an enterprise router 276. In the downlink direction, the CFAP 272 receives a voice frame from the IMS 224 via the enterprise router 276 and sends the same voice frame to all of the FAPs, i.e., FAP1 270 and FAPn 274 involved in the soft handoff. Typically, an enterprise FAP is a miniature base station that implements the CDMA base transceiver station function, the base station controller function, and the packet control function (PCF). Enterprise FAPs are grouped by clusters. Each cluster has a Controlling FAP. All enterprise FAPs within a cluster communicate with each other via a local enterprise IP backbone 278. Signaling protocols between the CFAP 272 and the individual enterprise FAPs 270, 274 have not yet been defined by 3GPP2. Voice paths between the CFAP 272 and the individual enterprise FAPs 270, 274 may be established by using the Real-Time Transport Protocol.
The converged CDMA IMS Femtocell system also includes a Femtocell Management System (FMS) 230 that provides femtocell operation, administration, maintenance, and provisioning (OAM&P) functions.
The converged CDMA IMS Femtocell system also includes a Femtocell Authentication, Authorization, and Accounting (AAA) module 232 that provides a FAP authorization function. The AAA 232 sends authorization policy information to the Security Gateway (SeGW) 234. The SeGW authenticates the FAPs and protects the IP core network 228 from security attacks (e.g., denial of service) initiated by FAPs.
The converged CDMA IMS Femtocell system also includes IMS 224, an IP Multimedia Network 226, and an IP backbone network 228.
The converged CDMA IMS Femtocell system also includes a Media Gateway Control Function/Media Gateway (MGCF/MGW) 222 that connects IMS 224 to the legacy CDMA core network.
The converged CDMA IMS Femtocell system also includes a Femtocell Convergence Server (FCS) 210. The FCS 210 is an IMS Application Server that provides internetworking between the FAP, the SIP environment of IMS, and the appropriate Mobile Access Protocol (MAP) 212 network elements.
The converged CDMA IMS Femtocell system also includes legacy CDMA core network elements, including: the public switched telephone network (PSTN) 218 and a public safety answering point (PSAP) 220; a mobile switching center (MSC) 216; a base station controller (BSC) and a base station transceiver system (BTS) 214, a home location register (HLR) and authentication center (AC) 202; a position determining entity (PDE) 204; a mobile positioning center (MPE) 206; and a message center (MC) 208.
The MSC 216 is connected to the PSTN switch 218 via a TDM trunk. The MSC 216 is connected to the MGCF/MGW 222 via a nailed-up TDM trunk. The MSC 216 interacts with the HLR 202 and the CS 210 using IS-41 signaling. The MSC 216 is connected to at least one BSC via a TI connection or an Ethernet connection. Each BSC controls at least one BTS 214 via a T1 connection or an Ethernet connection. An MSC 216 can be either a TDM mobile switching center or a “Soft Switch” MSC.
Referring now to FIG. 2C, the MSC 216 includes a radio access control function (RACF) subsystem 262, a call control function (CCF) subsystem 264, and a trunking subsystem 260. The RACF subsystem is responsible for paging, mobility management (e.g., location update), and handoff. The RACF subsystem 262 interfaces with the base station controllers 214 and 254, typically via either a T1 interface or an Ethernet interface. Base station controller 214 interfaces with base station transceivers 250 and 252 via T1 or Ethernet, and base station controller 254 interfaces with base station transceivers 256 and 258 via T1 or Ethernet. The mobile station 240 communicates with the base station transceiver having the highest signal strength at any given moment.
The RACF subsystem 262 performs handoffs in coordination with the base station controllers 214 and 254, the trunking subsystem 260, and the convergence server 210 via respective IS-41 interfaces.
The RACF subsystem 262 also interfaces with a home location register (HLR) 268 and a visitor location registry (VLR) 266. The HLR 268 contains subscriber information for the mobile stations of a plurality of subscribers. The VLR 266 contains information relating to all mobile stations which have registered with the MSC 216.
The CCF subsystem 264 is responsible for translating and routing calls. The trunking subsystem 260 is responsible for trunk connectivity with the PSTN 218, the MGCF/MGW 222, and other mobile switching centers.
Conventional systems include a capability to complete a handoff of an active communication connection from a femtocell to a macrocell, e.g., when a mobile user station moves from an indoor residential environment out into a generic macrocell environment. However, currently, the 3GPP2 femtocell standards and femtocell and convergence server vendors lack an efficient solution for handing off an active mobile user station (for example, a mobile user station that is on a call with another party) from a macrocell to a residential femtocell or to enterprise femtocells when the mobile user station moves into the femtocell coverage area. Consequently, a subscriber who is talking on a call via a macrocell base station and is moving to an indoor environment, such as a private residence, an enterprise, or a hotspot, will suffer from poor macrocell RF coverage in an indoor environment. Hence, there is a need for a method for efficiently performing a handoff from a macrocell to one or more femtocells that minimizes the likelihood of dropping the active communication connection during the handoff.